1. Field of the Invention
The present invention relates to a color image forming apparatus and method of forming a color image using an electrophotographic process.
2. Discussion of the Background
A so-called tandem-type method is used in a color image forming apparatus in which a color image is obtained by transferring by superposition respective color images formed with an electrophotographic processing section of the apparatus onto a recording sheet.
FIG. 1 is a side view of a color image forming apparatus of the tandem-type method. As shown in FIG. 1, a sheet conveying path 4 is provided for guiding a transfer sheet 1, as a recording sheet, from a sheet feeding section 2 through a sheet discharging section 3 in the color image forming apparatus. The sheet conveying path 4 includes a conveying belt 7 that is movably positioned between a belt drive roller 5 that is rotated by drive power from a drive power source (not shown) and a belt driven roller 6 coupled to the drive power source. Further, on the conveying belt 7, four electrophotographic processing sections 8Y, 8M, 8C, and 8K, for yellow, magenta, cyan, and black are respectively disposed in order. These electrophotographic processing sections 8 respectively include a photoconductive drum 9, as a photoconductive element, that contacts the conveying belt 7, as well as a charging device 10, an exposing device 11, a developing device 12, a transferring device 13, and a photoconductive element cleaner 14 each being disposed in order around the photoconductive element 9. In addition, the conveying path 4 is provided with a fixing unit 15 at a position just after the conveying belt 7, as shown.
Typically, the color image forming apparatus that has such a construction feeds an uppermost transfer sheet 1 from a sheet feeding section 2 towards the sheet conveying path 4, and is conveyed with the conveying belt 7. During the sheet conveying process, an image forming operation for each of the four colors is performed by each electrophotographic processing section 8, using electrophotographic processes, i.e., charging, exposing, developing, and transferring processes. A color toner image is transferred onto the transfer sheet 1, and fixed thereon by being heated and pressed with the fixing unit 15. This is a principle of image forming by the color image forming apparatus of the tandem-type method as shown in FIG. 1.
FIG. 2 illustrates from a perspective view the conveying belt 7 and respective drums 9. As will be discussed herein, a main scanning direction is indicated by a mark B, and a sub-scanning direction is indicated by a mark C.
Even though the color image forming apparatus of the tandem-type method has an advantage of high printing speed, the present inventors recognized that this conventional apparatus has a shortcoming in that an alignment of each of the colors is difficult to achieve and maintain. Therefore, for example, a slight positional deviation often occurs when a user or a service engineer moves a part of the electrophotographic processing section 8 from a proper position when removing a jammed sheet or repairing the apparatus, and this slight positional deviation causes a color deviation between the respective colors.
Several approaches to preventing the color image positional deviation have been proposed in recent years.
For example, as discussed in reference to FIGS. 3 and 4, an image positional deviation detecting method is disclosed in Japanese Laid-Open Patent Publication NO. 6-18796/1994. Image positional deviation detecting sensors 102, which include two CCD line sensors 101 (in FIG. 4), are positioned so as to face the conveying belt 7. Image positional deviation detecting marks 103 are formed on the conveying belt 7 by the electrophotographic processing section 8 before the image forming operation is performed. The detecting marks 103 are positioned in areas where the CCD line sensors 101 can read them such that an amount of image positional deviation corresponding to the electrophotographic processing sections 8Y, 8M, 8C, and 8K can then detected by reading the positional deviation detecting marks 103 by the CCD line sensors 101 as shown in FIG. 3. The image positional deviation detecting sensor 102 includes a light source 104 and a light collecting lens 105 for collecting and providing reflection light to the CCD line sensor 101, reflected by the conveying belt 7, which is emitted from the light source 104 as shown in FIG. 4.
However, as presently recognized, the image positional deviation detecting method disclosed in Japanese Laid-Open Patent Publication No. 6-18796/1994 has some problems in that parts costs are greater than desired due to the inclusion of the expensive CCD line sensor 102 or light collecting lens 105. Furthermore, focusing of the reflection light from the conveying belt 7 must be adjusted by the light collecting lens 105, and therefore the successful operation of the apparatus become troublesome.
In reference to FIGS. 5 through 8(b), and in light of the limitations of the above-mentioned method, a device is described in Japanese Laid-Open Patent Publication No. 6-118735/1994 as detecting the color image positional deviation using an inexpensive reflection-type optical sensor 204 composed of a light source 201, and a slit 202, and a light accepting element 203. Namely, V-shaped image positional deviation detecting marks 205 are formed on the conveying belt 7, and a leading edge and a trailing edge thereof are detected with two reflection-type optical sensors 204, as shown in FIG. 6. For example, in the case of detecting the image positional deviation between a black electrophotographic processing section 8K (FIG. 1) and a magenta electrophotographic processing section 8M (FIG. 1), two black lines K1 and K2 which compose each edge of the first V-shaped mark, two magenta lines M1 and M2 which compose each edge of the second V-shaped mark, a black line K3 which composes one edge of the third V-shaped mark, and a magenta line M3 which composes another edge of the third V-shaped mark are formed on the conveying belt 7, as shown.
FIG. 7 shows an example in which the magenta electrophotographic processing section 8M deviates in a sub-scanning direction. Namely, when the image positional deviation detecting mark 205 is detected with respective reflection-type optical sensors 204, an output signal from one side of the reflection-type optical sensor 204a (lower part of FIG. 7) is represented in FIG. 8(a), and another side of the reflection-type optical sensor 204b (upper part of FIG. 7) is represented by a diagram in FIG. 8(b). Thus, if a time difference between pulses based on a signal of one side reflection-type optical sensor 204 is not constant {FIG. 8(a)}, and a time difference between pulses based on a signal of another side reflection-type optical sensor 204 is constant {FIG. 8(b)}, an electrophotographic processing section 8 of a certain color is judged to have deviated in a sub-scanning direction.
In light of the above description regarding deviation in the subscanning direction, it is possible for deviations to occur in the main scanning direction. More particularly, when an electrophotographic processing section 8 of a certain color deviates in position along the main scanning direction, the timing of output signals from two reflection-type optical sensors 204a, 204b deviates. For example, if the image positional deviation detecting mark 205, composed of two black lines K1 and K2 which construct each edge of the first V-shaped mark, deviates upwards, it is assumed that a pulse based on the output signal of the reflection-type optical sensor 204b {FIG. 8(b)} precedes a pulse based on the output signal of the reflection type optical sensor 204a {FIG. 8(a)}. Therefore, the image positional deviation in the main scanning direction of the electrophotographic processing section 8 can be detected by detecting the output pulse timing of the respective reflection-type optical sensors 204a, 204b.
Since the invention disclosed in Japanese Laid-Open Patent Publication No. 6-118735/1994 has a construction which detects the image positional deviation detecting mark 205 using the inexpensive reflection-type optical sensor 204 composed of the light source 201, the slit 202, and the light accepting element 203, the parts costs are much less than the apparatus disclosed in Japanese Laid-Open Patent Publication No. 6-18796/1994.
However, in accordance with a detection aspect of the apparatus disclosed in Japanese Laid-Open Patent Publication No. 6-118735/1994, two reflection-type optical sensors 204 are required, and therefore, the parts cost is greater than that for an apparatus requiring a single detector, and the construction thereof becomes complicated due to the need to secure enough space for mounting the two reflection-type optical sensors 204.
Further, since the image positional deviation detecting mark 205 is formed on the conveying belt 7 with an electrophotographic process, toner is randomly scattered at an edge part E of the image positional deviation detecting mark 205. As shown in FIG. 9, this scattering of toner gives rise to a problem in that a sharply contrasted output signal cannot be obtained by the reflection-type optical sensor 204. Namely, an output waveform from the reflection-type optical sensor 204 has a gentle slope as shown in FIG. 10 which increases the difficulty of detecting a leading edge and a trailing edge of the mark 205.